Flow cytometry is an emerging technology that has become the method of choice of measuring the binding of monoclonal antibodies to cells. Early studies have established several candidate methods that might be used to regulate the actual flow cytometry devices. These methods not only can compare qualitative analyses of antibody binding, but can also provide quantitative measurements to directly compare devices from manufacturers. Other aspects of this collaborative project involve quality assurance measurements of these candidate methods among the individual laboratories of the Flow Cytometry Consortium, as well as selected outside laboratories involved in flow cytometry research. Early results suggest that of the devices may not be major factors for the source of variation. Rather it is theneed for a standard calibration curve, unified setup, compensation controls, whole blood controls and control of fixation. Flow cytometry technology is rapidly expanding as a clinical tool that is used to classify AIDS patient categories, to define the disease of AIDS, to follow surrogate endpoints in many diseases, to prepare bone marrow stem cells and to define clinical correlates of disease. Because of its widespread use in clinical diagnosis and treatment, it is imperative to insure that the technology is reproducible and that the measurements and performance of such machines can be calibrated to standards. Because this technology crosscuts products regulated by CBER, CDER and CDRH, an interagency Flow Cytometry Consortium has been formed to pool resources and to develop methods for quantitative flow cytometry standards, and methods of quality assurance. The Consortium members include four FDA centers, the NIH and the CDC. Calibration curves for CD4 and CD34 are being prepared.